Bring 8 cups of water to a boil in a large pot, then add the meat, garlic cloves, celery, onion half, and 1 tablespoon of salt.
Let the mixture simmer over medium heat for about 1 hour and 30 minutes, or until the meat is tender.
Once done, remove the meat from the pot and let it cool completely.
Once cool enough to handle, cut the meat into chunks and hand-shred it just enough to make processing easier.
Transfer the meat to a food processor and pulse 4–6 times, or until it’s coarsely shredded (avoid over-processing).
In a large mixing bowl, combine the shredded beef with mint, radish, onion, tomato, lemon juice, and salt.
Mix thoroughly.
Adjust the seasoning as needed and serve chilled with rice or on tostadas topped with avocado slices.